Hi guys, i found a solution (the solution?)
The problem i had was because of the unpacker (WinRar)! WinRar unpacked all files in uppercase and some files get different names (vmlinuz-2.6.29.3-slitaz -> VMLINUZ_.3_S) while unpacking the .iso file. Then i used 7zip, copied the files to usb and everything works like a charm!
If you already copied files to the usb disk, you have to delete them first, overwrite it won't work and the files will get the old uppercase name, if you do. You have to delete them first manually!
I hope this helps some of you!